1. Field of the Invention
The present invention relates to a laser recording apparatus for printing image data on a recording medium by controlling an emission level of a laser diode according to the level of an image signal input.
2. Description of the Prior Art
It is well known that with a laser diode the output level of the laser beam may be modulated by modulating the electric current for driving the laser diode. A laser beam printer has been proposed which utilizes the above fact and carries out recording with tones by inputting image data in an analog signal to the laser diode, driving the laser diode, and controlling the output level of the laser diode.
Generally, such a laser beam printer employs a laser diode that emits a laser beam having a single wavelength since it is easy to converge the laser beam output by the laser beam. With the laser diode that emits a single wavelength laser beam, however, the emission wavelength is variable with even slight temperature variations, which results in noise known as mode hopping noise. The temperature and output level causing such noise vary from diode to diode even in the same lot. A further inconvenience is encountered where the laser diode is used as light source for the printer. In this case, light reflected by optical elements disposed on an optical path travels backward to cause periodic noise called optical feedback induced intensity noise. It is well known that the reflected light tends to amplify the mode hopping noise. The mode hopping noise and the optical feedback induced intensity noise markedly deteriorate the image quality of the printer.
To deal with these noises characteristic of the laser diode which provides a single wavelength emission, methods of superimposing a high frequency current of 500 MHz to 1 GHz (typically 700 MHz) on a direct current for driving the laser diode have been proposed in the Japanese patent publication No. 59-9086 and the Japanese patent applications laid open under Nos. 59-171037, 60-35344, 60-170041, 60-192377 and 56-83089.
According to the proposed technique, the laser diode is driven at the cycle of the high frequency current in such a way that the driving current for the laser diode is below the threshold level, thereby to periodically maintain a spontaneously emitting state wherein the laser diode emits in a wide wavelength region immediately before the laser emission.
The above prior art technique is based on the fact that, since the mode hopping noise and the optical feedback induced intensity noise are the phenomena attributable to the single wavelength emission of the laser diode, they may be eliminated by allowing the laser diode to emit light spontaneously in a wide wavelength region at a high frequency cycle. This technique is highly reliable with regard to stabilization of the laser diode output, but may produce only half of the intended effect depending on the construction of the laser diode used unless matching is effected with care. Furthermore, electromagnetic radiation noise can be produced unless a sufficient shielding treatment is provided for a high frequency wave generator and a laser diode light source portion.
The density adjustment for the above laser beam printer is conventionally carried out by rotating a beam splitter for example. FIG. 1 of the accompanying drawings illustrates a laser beam printer for which this method is executed. Reference number 1 indicates an analog input signal applied to a laser diode drive circuit 2, number 3 indicates a laser diode, and number 4 indicates a laser beam emitted from the laser diode 3 which is divided by a beam splitter 5 into an image forming beam 6 and a monitoring beam 7. The image forming beam 6 travels through a further beam splitter 8 and an optical system 9 to form an image on an image forming surface. The monitoring beam 7 is subjected to photoelectric conversion at an optical sensor 11 for negative feedback to the laser diode drive circuit 2.
The further beam splitter 8 is rotatable on an optical path. Assuming that the beam splitter 8 is rotated by an angle .theta. from an original position, the power of the laser beam which passes through the beam splitter 8 varies along a curve expressed by cos.sup.2 .theta. as shown in FIG. 2. This is because the transmission rate varies with the rotation of a polarization type beam splitter since the laesr diode has a P-polarized characteristic. The rotation of the beam splitter is controlled by software, and enables power range switching and density adjustment on the image forming surface.
The known density adjusting method noted above requires a mechanical device for rotating the beam splitter, and therefore gives rise to the problem of rusting and wear due to the material forming the mechanical device, environmental conditions and the like.
When the beam splitter is rotated by eccentricity, there arises the problem of displacing the optical axis and marring the image. This problem is notable particularly with the type of color laser beam printer which forms an image by superimposing a plurality of beams.